1. Field of the Invention
This invention relates generally to detectors that are very sensitive, and more particularly to a detection device using stochastic resonance to increase signal-to-noise ratio.
2. Description of the Related Art
In the context of this discussion, the word "stochastic" refers to a random noise which is induced in or inherent in the system which is the subject of the invention.
Stochastic resonance (SR) can be described as a statistical process in which a combination of noise and a weak signal induces transitions between the stable states of a bi-stable or multi-stable system. Such a system has been described by McNamara et al., Theory of Stochastic Resonance Phys. Rev. A, Vol. 39, No. 9, pp. 4854-4869 (1989). The principle of stochastic resonance can be most easily understood in the context of a simple, one-dimensional, bi-stable potential which is weakly modulated by an external periodic signal. The effect of an external input is to alternately raise and lower the level of the potential wells with respect to the effective barrier height between the wells. When noise is added to the system, a particle residing in one of the wells can move to the other by stochastic activation. More precisely, in the presence of the weak external modulation, the addition of noise to the system will cause the transitions between states to become more coherent with the signal. It is this periodic modulation of the transition rate which constitutes the coherence between the signal and the system response. This is a now well understood phenomenon, at least in research circles.
As a result of this behavior, under some conditions the coherence between the bi-stable or multi-stable system response and the weak modulation signal can actually be increased by increasing the noise in the system. In other words, in some types of multi-state systems, one can actually improve the signal-to-noise ratio (SNR) of the output of the system by increasing the noise.
For a fixed level of input noise, there can also be a maximum in the SNR for certain internal control parameters. This has important implications for any device based on SR effects, since one does not then need to adjust an external noise source in order to maximize the SNR for a particular input signal.
Stochastic resonance was first observed experimentally in 1983 by Fauve and Heslot, Stochastic Resonance in a Bistable System, Phys. Lett. Vol. 97A, No. 1, 2, pp. 5-7 (1983). By measuring the power spectrum of the device as a function of the noise intensity at the input, Fauve and Heslot showed that the SNR of the device went through a maximum at a particular noise intensity.
A more dramatic demonstration of stochastic resonance was seen in the 1988 ring laser experiments by McNamara et al. as reported in Observation of Stochastic Resonance in a Ring Laser, Phys. Rev. Lett., Vol. 60, No. 25, pp. 2626-2629 (1988). In this study the bi-stability was the result of the degeneracy associated with two counter-rotating laser modes. These experiments demonstrated an improvement in SNR of nearly 12 db for this particular apparatus. Stochastic resonance has also been demonstrated in a detuned electron paramagnetic resonance apparatus, in which certain combinations of the paramagnetic sample and the cavity resonance resulted in bi-stable operation. [Gammaitoni et al., Observation of Stochastic Resonance in Bistable Electron-Paramagnetic-Resonance Systems, Phys. Rev. Lett., Vol. 67, No. 13, pp. 1799-1802 (1991)].
In an array of globally coupled, weakly nonlinear oscillators with a linear mean field interaction, the stochastic resonance effect was shown theoretically to be greatly enhanced over what would be expected for a single isolated element of the network. This was shown by Jung et al., Collective Response in Globally Coupled Bistable Systems, Phys. Rev. A, Vol. 46, No. 4, pp. R1709-R1712 (1992). Subsequent experiments using an array of Schmitt triggers verified aspects of this behavior in a physical system as explained by Pantazelou et al., Noise Sampled Signal Transmission in a Array of Schmitt Triggers, Proc. 12th International Conference on Noise in Physical Systems, edited by Handel, American Institute of Physics Conference Proceedings, Vol. 285, pp. 549-552 (1993). In another manifestation of a globally coupled array of nonlinear oscillators, it has been shown theoretically that weakly nonlinear oscillators with nonlinear couplings can also exhibit stochastic resonance effects; Bulsara et al., Stochastic Resonance in Globally Coupled Nonlinear Oscillators, Phys. Rev. E, Vol. 47, No. 5, pp. 3734-3737 (1993) and Bulsara et al., Single Effect Neuron: Dendritic Coupling Effects and Stochastic Resonance, Biol. Cybern. 70, pp. 145-156 (1993). In this arrangement there is a central reference element and an array of globally coupled elements. If the time scale for relaxation of the reference oscillator is much longer than that for the array, the dynamics of the reference oscillator can be separated from the array and SR effects can be observed.
Despite the large amount of research done to demonstrate the existence of SR effects and to prove theoretic hypotheses relating to physically occurring phenomena in nature, practical applications of SR phenomena to a physical measurement problem or to the design of a device suitable for acting as a detector has not previously been accomplished.